Abstract Noise is the number one occupational and environmental hazard, and noise- induced hearing loss (NIHL) is one common form of sensorineural hearing deficit, only second to age-related hearing loss. Currently no drug preventing NIHL is approved by the United States Food and Drug Administration (FDA). One major challenge for the introduction of such interventions is the need to identify appropriate human populations available for evaluating the effectiveness of new otoprotective agents. In this STTR Phase I project, we propose to determine the extent of hearing loss in unoperated ears of patients after drilling during skull-based surgery. Based on our preliminary studies, we have reduced the variability of electrocochleography for detecting hearing loss. By working with our collaborators at Washington University School of Medicine in St. Louis, in two out three patients, we have detected temporal hearing threshold shifts in unoperated ears after the skull-based surgery. A recent seminal discovery in animals revealed that, even without permanent threshold shift, a loss of synapses between inner hair cells and spiral ganglion neurons in the inner ear is the primary pathology for NIHL. This form loss of synapses is called cochlear synaptopathy, or hidden hearing loss. So far, no longitudinal studies have been reported to confirm the existence of hidden hearing loss in people although two human studies implicated possibility of cochlear synaptopathy by comparing groups with different risks for noise exposure. Here, we will continue to recruit qualified patients scheduled for skull-based surgery. Functional assays, such as audiometric testing, electrocochleography, distortion product otoacoustic emissions, and the Words-in-Noise test with multi-talker babble will be repeated on their unoperated ears before the surgery, one day and one-month post- surgery. By the project?s end, we expect to quantify the extent of both temporary and permanent threshold shift in patients; and confirm the existence of hidden hearing loss in a certain percentage of these volunteers. The main innovative aspect of this study is to identify a new population with unavoidable noise exposure under a laboratory-like setting using improved functional assays. If successful, In the STTR or SBIR Phase II project, we will carry our FDA-approved clinical trial (NCT02049073) based on this population. Thus, this project will not only identify a new population for future clinical studies of new otoprotective agents, but also the first epidemiological study of hidden hearing loss in this population. Our long-term goal is to develop an effective medical intervention for NIHL.